With the development and spread of office copiers or copying machines and facsimile, it has been strongly desired to provide a practical means to inhibit the copying or confidential documents or copyrighted materials without permission.
In order to overcome the foregoing problem, several methods have already been proposed. Among the conventional methods is the one disclosed in U.S. Pat. No. 3,597,082. This method uses a photochromic paper which temporarily changes color to substantially match the color of data during the exposure step of the copying process, preventing the formation of clear copies therefrom. Another method is such that data which must be prevented from being copied are printed on a sheet having a specific background by the use of a specific ink. Image areas printed with the ink are reflective in a certain spectral range and cannot be distinguished from nonimage areas, i.e., the masking background, by a copying machine. On the other hand, within another spectral range, the image areas and nonimage areas are distinguishable from each other. By utilizing the foregoing properties, the method is intended to prevent copying. In accordance with the two methods, copying can be inhibited only when specific types of copying machines are employed; they cannot prevent copying using a so-called white-light copier in which light-sensitive materials coated with zinc oxide are used.
Furthermore, in accordance with the foregoing method, it is necessary to print images using a precisely selected color.
It is therefore desired to develop an apparatus which is not seriously influenced by the color of images, and which makes it possible to prevent copying when using any type of electrostatic copying machine (including the white-light copier) or facsimile.